This invention relates generally to the field of offshore platforms used in hydrocarbon exploration and/or production. More particularly, the invention pertains to the erection of such platforms utilizing an integrated deck installation and transport system.
Exploration and production of hydrocarbon reserves in arctic offshore regions present unique challenges. Starting in the late 1970""s certain offshore hydrocarbon reservoirs in arctic regions were developed by installing exploration and production equipment on man-made islands. These islands were constructed of gravel, sand, or dredged seabed fill material and were used in relatively shallow waters (approximately 50 feet or less) close to the shore. After construction of such an island, drilling rigs and equipment were brought to the site either by helicopter, by trucking over the surrounding ice during early winter, or by barge during the warmer months. These systems were cost-effective where ease of access from land, suitable fill material, and stable ice conditions existed. Examples of these man-made islands are described generally in Galloway, Scher, and Prodanovic, xe2x80x9cThe Construction of Man-Made Drilling Islands and Sheetpile Enclosed Drillsites in the Alaskan Beaufort,xe2x80x9d 1982 Offshore Technology Conference (OTC) Paper No. 4335, and Agerton, xe2x80x9cConstruction of an Arctic Offshore Gravel Island in 39 ft of Water During Winter and Summer,xe2x80x9d 1983 OTC Paper No. 4548.
For operations in water depths of greater than 50 feet, island fill volumes, and therefore costs, become excessive due to the natural slopes of the fill material (e.g. 1:3 for gravel, 1:12 for sand/silt). To reduce island fill volumes, the Caisson Retained Island (CRI) was developed. Steel and concrete CRIs provide much steeper slopes than the natural fill material. Once installed at the site, either on the sea bottom or on a submerged berm, the caissons are filled with dredged material. These systems are described generally in Fitzpatrick and Denning, xe2x80x9cDesign and Construction of Tarsiut Island in the Canadian Beaufort Sea,xe2x80x9d 1983 OTC Paper No. 4517 and Mancini, Dowse, and Chevallier, xe2x80x9cCaisson Retained Island for Canadian Beaufort SeaGeotechnical Design and Construction Considerations,xe2x80x9d 1983 OTC Paper No.4581. After construction of the CRI, drilling equipment is delivered to the working surface by either helicopter or barge.
As the desired water depth for exploration and production drilling continued to increase, man-made and caisson-retained islands became technically and economically infeasible. Due to the severe, dynamic ice loads in water depths greater than 60 feet and the relatively short open-water construction season, a number of new drilling concepts were developed in the early 1980""s to suit the demanding environment. Examples of these new concepts include the Concrete Island Drilling System (CIDS), the Single Steel Drilling Caisson (SSDC), and the Mobile Arctic Caisson (MAC). These systems are described generally in: Gijzel, Thomson, and Athmer, xe2x80x9cInstallation of the Mobile Arctic Caisson Molikpaq,xe2x80x9d 1985 OTC Paper No. 4942; Masonheimer, Deily, and Knorr, xe2x80x9cA review of CIDS First-Year Operations,xe2x80x9d 1986 OTC Paper No. 5288; and Masterson, Bruce, Sisodiya, and Maddock, xe2x80x9cBeaufort Sea Exploration: Past and Future,xe2x80x9d 1991 OTC Paper No. 6530. These systems are generally large monolithic systems constructed and fully outfitted with drilling equipment in a temperate environment and then towed to the desired arctic location. Because of their large size, these systems are subject to comparably large ice and wave loads, resulting in increased design and construction cost to address those loads.
The CIDS, SSDC, and MAC systems have been successfully deployed for exploratory well drilling during the relatively short drilling season in the Canadian and Alaskan Beaufort Sea. However, these concepts may not be suitable for general year-round drilling without ice management and also are not truly mobile compared to conventional jack-up rigs, drill ships, and semi-submersibles. Use of these systems in greater water depths and/or more severe ice conditions (i.e. year-round operations) requires the construction of costly man-made berms in conjunction with expensive foundation and mooring systems. As a consequence, development of hydrocarbon reserves in certain arctic regions may be uneconomic using these systems due to the limited number of wells that can be drilled during the drilling season.
Conventional jack-up drilling rigs permit quick installation and removal of equipment at a drill site, but are structurally incapable of withstanding ice loads without significant strengthening thus severely limiting their usefulness in arctic regions. U. S. Pat. No. 4,648,751 (Coleman) discloses the use of a U-shaped barge for the delivery and installation of an integrated deck system to a single-column offshore substructure. The integrated deck is supported and transported on jack and leg assemblies mounted on the barge. Upon arrival at the substructure, the jacks are used to lift the integrated deck above the top of the substructure and the U-shaped barge is maneuvered to position the deck over the substructure. The jacks are then lowered to set the deck on the substructure and the barge is removed. Although the system disclosed in Coleman permits delivery of an integrated deck system to a single-column substructure capable of withstanding the arctic environment, installation or removal of the deck is dependent on the availability of a U-shaped barge of the correct configuration, size, and capacity.
Persons skilled in development of offshore hydrocarbon resources will readily understand the economic incentives for low-cost drilling platform systems. The use of integrated deck systems that are assembled remotely and then transported to the fmal offshore installation site may reduce overall erection costs regardless of temperature and weather conditions at the site. For certain arctic regions, this incentive is magnified if such a deck system can be used in combination with a small, single-column, ice-resistant substructure. Furthermore, it would be desirable to have a mobile drilling and production system capable of year-round drilling operations even in severe arctic conditions. Also, offshore platform systems capable of quick installation, removal, and relocation would be particularly advantageous in arctic regions subject to fast-changing and extreme weather and severe ice conditions. The present invention provides a system capable of meeting these needs.
The present invention includes an apparatus and a method for installation of a deck on to an offshore substructure. The apparatus can be configured either for floatation and transportation or for fixed hydrocarbon drilling operations. The invention is useful in any offshore environment but is particularly suited for economic development of offshore hydrocarbon reserves in severe arctic regions.
The apparatus is self-floating and includes a deck, at least one pontoon, and at least one lifting support connecting each pontoon to the deck. The one or more pontoons have sufficient composite buoyancy to provide the apparatus with a net positive buoyancy. In the floatation configuration, the deck is supported by the one or more lifting supports, which are in turn supported by the pontoon(s), and the entire weight of the apparatus rests on the water. The lifting supports are typically in a compressed position so that the deck is relatively close to the pontoons and the water, and the apparatus is sufficiently buoyant and stable for transport on the open water.
In the operation configuration, the entire weight of the apparatus is supported by the offshore substructure upon which the deck has been installed. The weight of the one or more pontoons is supported by the one or more lifting supports which are in turn supported by the deck. In the operation configuration, the lifting supports are typically in a compressed or retracted position so that the pontoons are free from contact by waves or ice. In some embodiments for improved seismic response, one or more of the pontoons are removed from the lifting supports after installation of the deck on the substructure. In other embodiments, the pontoon(s) provide floatation during transportation and serve as additional deck work area during operation. In yet other embodiments, the deck is configured to provide additional floatation during transportation.
Installation of the apparatus on to an offshore substructure having an upper end adapted to support the weight of the deck and the pontoons is accomplished by transporting the apparatus in the floatation configuration to a location proximate to the substructure. Preferably, the upper end of the substructure is also elevated above the surface of the water. The deck is then elevated an amount sufficient to permit positioning of the deck over the upper end of the substructure by extending the lifting supports. The apparatus is then moved on the surface of the water, with the lifting supports extended, to position the deck at a selected location over the upper end of the substructure. After positioning, the lifting supports are retracted until the weight of the apparatus is transferred from the water to the substructure. The lifting supports are further retracted to lift the pontoons to a desired elevation above the surface of the water.